EAS and RFID systems are typically utilized to protect and track assets. In an EAS system, an interrogation zone may be established at the perimeter, e.g. at an exit area, of a protected area such as a retail store. The interrogation zone is established by an antenna or antennas positioned adjacent to the interrogation zone. The antenna(s) establish an electromagnetic field of sufficient strength and uniformity within the interrogation zone. EAS markers are attached to each asset to be protected. When an article is properly purchased or otherwise authorized for removal from the protected area, the EAS marker is either removed or deactivated.
If the marker is not removed or deactivated, the electromagnetic field causes a response from the EAS marker in the interrogation zone. An antenna acting as a receiver detects the EAS marker's response indicating an active marker is in the interrogation zone. An associated controller provides an indication of this condition, e.g., an audio alarm, such that appropriate action can be taken to prevent unauthorized removal of the item.
An RFID system utilizes an RFID marker to track articles for various purposes such as inventory. The RFID marker stores data associated with the article. An RFID reader may scan for RFID markers by transmitting an interrogation signal at a known frequency. RFID markers may respond to the interrogation signal with a response signal containing, for example, data associated with the article or an RFID marker ID. The RFID reader detects the response signal and decodes the data or the RFID marker ID. The RFID reader may be a handheld reader, or a fixed reader by which items carrying an RFID marker pass. A fixed reader may be configured as an antenna located in a pedestal similar to an EAS system.
It is advantageous for both EAS and RFID systems to have a sufficiently strong and uniform magnetic field within the interrogation zone in order to provide for reliable marker detection. However, if the magnetic field radiates at significant strength levels beyond the interrogation zone, problems can occur. For instance, the associated EAS or RFID system may not comply with regulatory requirements that restrict the level of the magnetic field at various distances from the antenna(s).
In order to establish a strong field near the antenna, i.e. in the interrogation zone, and a diminished field far away from the antenna to comply with regulatory requirements, field canceling arrangements for loop antennas have been developed. Such a field canceling arrangement may include a nested loop configuration where an inner loop antenna is nested within an outer loop antenna in a common plane. The outer loop antenna and inner loop antenna are designed so that the magnetic fields from each of the loops are equal and opposite at a distance far away from the antenna causing the fields to cancel.
However, the configurations utilized for field canceling with loop antennas do not directly translate to magnetic core antennas. This is due to the differences in the construction of the antennas, the field concentrating characteristics of high permeability core materials, and the major differences in the shape and gradients of the resulting magnetic field. In addition, the nested loop construction for loop antennas is simply not possible with magnetic core antennas.
In addition, it is desirable to have the magnetic field sufficiently strong across the entire plane of the interrogation zone in many orientations since markers are often directionally more sensitive in one orientation when aligned with the orientation of the magnetic field. Areas of a weak magnetic field in the interrogation zone may develop in various orientations in certain regions based on the particulars of the system. Such areas are referred to herein as “null zones” because the magnetic fields emanating from the antennas cancel to form a “null” in the magnetic field. Such null zones degrade the performance of the system as a marker passing through a null zone in a certain orientation may not be properly detected.
Magnetic core antennas contribute to the presence of null zones since they establish fields having regions where the magnetic field vectors are perpendicular to the orientation of the marker. In addition, in systems utilizing multiple core antennas, there are also certain regions where the magnetic field vectors from adjacent core antennas cancel one another.
Accordingly, there is a need for a field canceling arrangement utilizing magnetic core antennas. There is also a need in the art for an apparatus and method of reducing null zones created in a magnetic core antenna system.